In the transportation of materials by truck between a loading point and a destination, it is common practice to maintain the compartment closure in locked condition by use of a padlock which engages a suitable hasp or the like to prevent theft or tampering with the contents while in transit. In some instances the door is locked at the loading point by authorized personnel and unlocked at the destination by personnel other than the driver, who is not provided with a key, thus obviating suspicion that the driver has unlocked the closure during transit. Such locks are relatively easy to remove by cutting or jimmying. While there is no certainty that the driver may have removed the lock in such manner during transit, he is under a heavy burden to explain why the truck was left unattended at any stopping point to permit theft or pilfering of the contents. As will be apparent, this suspicion could be obviated if a locking system were provided which is jimmy-proof or otherwise tamper-proof by the driver or any other person than the driver.
Electrically operated bolts for locking closures, such as doors, have long been known as exemplified by U.S. Pat. No. 603,321 to Carlton (1898) and U.S. Pat. No. 947,866 to Taylor (1910), these differing in the manner in which control of the bolt was desired. In a further advancement in the art, as exemplified by U.S. Pat. No. 1,558,707 to Milligan (1925), it was recognized that a need existed for a solenoid operated bolt which could be positively latched in its locked and unlocked positions. In this connection, selective energization of two solenoids was apparently under manual control of an operator and one of the solenoids could remain energized after moving the bolt to desired position unless the operator opened a suitable switch. This inadvertence to open the circuit was not only a waste of electrical energy but also could burn up the insulation of a field winding unless it was designated for continuous energization. It is believed apparent that if the circuits could automatically be opened after throwing the bolt to one or the other of its two positions any inadvertence in opening a manually operated switch would be obviated.
Another disadvantage of the Milligan construction appears to reside in the manner in which the bolt locking latch is actuated by magnetic attraction toward the solenoid cores, the flux density in the magnetic circuit being relatively weak due to the large air gap therein. As will be apparent, just prior to release of a latch, it is bucking movement of a core and, hence, the attractive force must be in excess of the axial force on the core. It is believed apparent, accordingly, that it would be advantageous to move the latch under control of an independent actuator so that the actuator could be designed to perform its unlatching function independent of the core moving function and not dependent upon some partially common flux density therebetween.